Internal deformation of the Gorda Plate observed by hydroacoustic monitoring

On August 29, 1991, the National Oceanic and Atmospheric Administration VEN TS Program began monitoring the northeast Pacific Ocean plate boundaries us ing continuously recorded data from the U.S. Navy's Sound Surveillance Syst em (SOSUS) hydrophone arrays. Routine detection and location of small (2.4 less than or equal to M less than or equal to 3.5) earthquakes revealed a N NE-SSW trending band of seismicity through the center of the Gorda Plate. T he majority of the earthquakes in the band were not detected by land-based seismic networks. This band of microearthquakes was active from the initiat ion of SOSUS monitoring in August 1991 through July 1992, at a rate of simi lar to 20 events per week. Since August 1992, however, the activity has eff ectively ceased. Some of the microearthquakes in the band are likely afters hocks to three large (M-W > 6) mid Gorda Plate earthquakes that occurred 1- 2 months prior to the beginning of SOSUS monitoring. The hydroacoustically detected events in the band, however, do not occur in the proximity of the M-W > 6 events, and the activity level increased several months afterward, suggesting the microseismicity band represents a more general pattern of de formation within the Gorda Plate. The cessation of the midplate activity fo llows the occurrence of a large earthquake sequence in the adjacent Cascadi a Subduction Zone in April 1992. It is proposed that the band of microseism icity and the three large midplate events reflect an accumulation of stress in the Gorda Plate, with the eventual termination of this pattern due to s tress reduction associated with movement in the adjacent subduction zone. S tress field modeling of the region yields results consistent with this inte rpretation.